Due to wear, environment, age, misapplication, or manufacturing flaws, control valves used in the process industries can perform improperly or fail. When they perform improperly, the result is often lower yield of the process. If they fail it will often shut the process down. In a large refinery or chemical plant or power plant either of these outcomes can be very expensive. Plant operators have been interested in finding ways to detect problems in valve operation before the problem manifests itself in ways that hinder plant production.
Some common problems that occur in valves are: seat wear or damage, stem damage, stem corrosion, stem wear, packing wear, linkage problems/failures, actuator spring problems/failures, and actuator diaphragm failures. Many of these problems cannot be definitively identified except by disassembling the valve and examining the parts. This is a common practice in valve diagnostics, but is an expensive solution.
In recent years, operators have begun to infer many of the common problems from tests that they can make with the valve fully assembled and still in the process line (although when the process is not running such as during a plant shutdown). Valve “testers” have been built which attach sensors to the valves and make measurements. The most common measurements include positioning error, accuracy, response time, overshoot, spring range, and friction. To perform these tests, sensors must be attached to the valve and the valve must be stoked through much of its range. This prevents these tests from being performed during plant operation.
The use of data acquisition systems to gather valve operation data has been available for some time but has not been widely used for specific valve diagnostic applications. More often data acquisition system data has been used to evaluate overall process operation and in process tuning. Valve setpoint and valve position have been available using SCADA systems or the control system for some time.
More recently, some of the data acquisition has been moved to the valve itself in the form of smart positioners. Instead of the valve setpoint and position being measured from analog signals sent back to the control room, these measurements are made at the valve and sent back to the control room in digital form using one of several standard communications protocols. Some of the vendors of the smart positioners have included diagnostic applications in the firmware of the positioners allowing the positioner to control the valve to perform some diagnostic tests. These tests include response time tests, step response tests, actuator signatures, and friction analysis. With a few exceptions, however, these tests must still be performed when the process is not running.
There has been considerable interest in tests that can be performed when the process is still running. At least one vendor has built some tests in the firmware of their positioner to gather diagnostic information. This vendor requires the use of a special version of the positioner to work with software in the control room to perform these tests and can provide information about valves during the normal operation or during operation where the valve is artificially moved but only a small amount which would be tolerated by the running operation.
Thus, there is a need in the art for a method and system for performing valve diagnostics during normal valve operation.